Inter-working between a circuit-switched network and Internet telephony requires that the call be routed via an appropriate signaling and media gateway. This gateway, which appears as an end device to both involved technologies, translates the signaling messages and the media streams between the two involved signaling protocols and media formats. Optimization of the selection of one particular gateway for a call essentially means to best meet the selection policies of the involved parties, i.e., caller, callee, network and gateway providers. These policies are typically based on criteria such as: minimizing triangle routing; minimizing the circuit-switched call leg (e.g. to minimize cost); and maximizing the provisioned Quality of Service (which can be viewed to mean to minimize the Internet call leg); and gateway capacity and load balancing.
For inter-working calls from an Internet telephony user agent to a fixed-wire PSTN network the originating technology, i.e. the originating Internet telephony network, knows the location of the party to be called by the structure of its E.164 phone number. Consequently, the gateway selection process can be based on knowledge about the entire resulting call path, and can thus be optimized towards the selection criteria mentioned above.
When the location of the called party is not reflected by its address—because the terminating technology supports device mobility (cellular phone networks) or user mobility (e.g., SIP—Session Initiation Protocol)—the originating technology must perform the gateway selection without knowledge of the called party's location, and thus without knowledge of the resulting call path. This is sub-optimal as many of the above mentioned selection criteria require prior knowledge of the resulting call path. As an example, assume a SIP user calls a GSM phone and the gateway selection criterion is ‘minimize the PSTN call leg’. This criterion is best met by selecting a gateway that is close to the GSM callee; but as the actual location of the callee is not known to the originating SIP network, this selection cannot be made.
The above scenarios cannot be dealt with by means of existing gateway selection techniques. Instead, there is a need for a more comprehensive concept of mobility management.